1. Technical Field
The present invention relates to a method of designing a progressive power lens for sight correction.
2. Related Art
The progressive power lens has an upper distance-use area for viewing a distance, a lower near-use area different in refractive power from the distance-use area and for seeing a near, and a progressive area having a refractive power progressively changing between those, in at least any one of the two refracting surfaces at the object and eye sides structuring its spectacle lens. Various attempts have been made on the progressive power lens in an effort to improve the optical capability. Attentions are paid for, one of those, a progressive power lens using an aspheric design. This is assumed the same condition as wearing spectacle lenses over the eyes, to compute a dioptric power, astigmatism, prism, etc. by ray tracing, thereby compensating for the regions where errors are to occur in spherical design.
Incidentally, the progressive refracting surface itself is of an aspheric surface because it, in nature, is of a smooth connection between the spherical surfaces for distance and near uses that are different in curvature in one surface. However, the aspheric design of a progressive power lens, referred herein, means not of a navel point in a mathematical sense even in a region where the curvature of a progressive refracting surface is constant such as at a distance-use center or at a near-use center.
The progressive power lens using such aspheric design is disclosed in the below JP-B-2-39768, wherein there are provided effects including astigmatism reduction, lens thickness reduction, etc. as compared to the spherical design. However, the aspheric design described in JP-B-2-39768 is considered far from the optimal aspheric design. For this reason, the present inventor has proposed, in the below JP-A-2000-66148, a spectacle-lens designing method that the optimal aspheric design can be applied throughout the lens including the progressive area by a simple-and-easy lens design.
However, in the designing method proposed in JP-A-2000-66148 by the present inventor, the distance-use and near-use areas of the progressive power lens are designed aspheric due to the consideration that those are regions each having a constant refractive power. Nevertheless, the spectacle lenses actually produced, at nearly 70 percent, are added with astigmatism-correcting refractive powers. It is the current situation that those, in majority, are the spectacle lenses each having a refracting surface provided with a combined refracting surface where progressive and astigmatic refractive surfaces are combined together. The designing method proposed in JP-A-2000-66148 does not take account of a cylindrical power, and hence not optimized in correcting, by an aspheric design, a combined refracting surface where the progressive refracting surface is combined with an astigmatic refracting surface.